1. Field of the Invention
The present invention is directed to a biomagnetic measuring installation having a measuring cryostat and a patient support table, the installation being seated on a foundation base.
2. Description of the Prior Art
A very early and thus imperfect measuring installation is described in the article "A Study of the Vector Magnetocardiographic Waveform," by Rosen et al appearing in IEEE Transactions on Biomedical Engineering, Vol. BME-22, No. 3, May 1975 at pages 167-170. In this structure, the patient support and the measuring installation are simply placed on a floor. Such an arrangement, however, results in excessively high noise signals due to mechanical vibration.
A more stable measuring installation is described in "The Berlin Magnetically Shielded Room, Section A: Design and Construction," by Mager appearing in Biomagnetism, 1981. This shielded room is also described in the article "Laesst Hirnstroeme am Magnetfeld messen," by Baier appearing in Elektrotechnik, Vol. 63, No. 24, December 1981 at pages 24-25. This biomagnetic measuring installation, which has a shielding consisting of a number of layers of mumetal is shown on page 74 of the Biomagnetism article. The floor of the measuring chamber is connected to a foundation or base through columns. The cryostat and the patient support are arranged on the floor of the measuring chamber on a stand. In this arrangement, vibrations at the measuring cryostat, which may falsify the measured result, are possible, given movements from outside of the chamber, such as jolts, or movements of the patient within the chamber on the patient support. A movement of the measuring coil by only a few micrometers significantly deteriorates the measured result in the measuring cryostat, which is generally a superconducting Josephson junction magnetometer, also known as SQUID. This is because the measuring coil registers noise signals in addition to the signals from the body of the patient. These noise signals are caused by movements of the measuring coil and the patient relative to each other, or by movements of the measuring coil and the magnetic field of the environment relative to each other. A measuring cryostat of the type under consideration herein is described, for example, in "Biomagnetismus, Signale aus dem Koerper," by Hohnstein in Bild der Wissenschaft, Vol. 8, 1986, page 79.
Various attempts at solving the problem of protecting a biomagnetic measuring installation from vibrations have been proposed in recent years. A satisfactory solution, however, was found.
A vibration shielding chamber is described in "Design, Construction and Performance of a Large-Volume Magnetic Shield," by Kelhae et al, appearing in IEEE Transactions on Magnetics, Vol. MAG-18, No. 1, January 1982 at pages 260-270. The entire chamber is placed on cement blocks. While this has some effect in minimizing vibrations from the exterior of the chamber, vibrations caused inside the chamber (for example, when the patient or the attendant moves) are not suppressed.
Another type of measuring installation is disclosed in "Application of Superconducting Magnetometers to the Measurement of the Vector Magnetocardiogram," Wikswo Jr. et al, IEEE Transactions on Magnetics, Vol. MAG-13, No. 1, January 1977, pages 354-357. This installation is resiliently suspended together with the measuring cryostat and the patient support. A magnetically shielded room is described in "Installation of a Biomagnetic Measurement Facility in a Hospital Environment," Bercy, Biomagnetism 1981 at page 105. The magnetically shielded room is supported on the foundation by damping elements. The room is thus elastically seated, and thus produces an increase in vibrations in the region of the resonant frequency, and a reduction in vibrations above the .sqroot.2.sup.th resonant frequency. Very low resonant frequencies currently obtainable are in the range between 1 and 5 Hz. An oscillatory increase at low frequencies can therefore occur on the basis of frequency components due to external jolts or due to movements of the patient, such as heart activity or breathing activity of the patient.
It is preferable to mount the cryostat as rigidly as possible. This is because the cryostat must be disposed immediately above the patient. If damping elements, such as pneumatic dampers, are used to mount the cryostat, such damping elements may fail. The cryostat is filled with liquid helium, and has a very thin walled base adjacent the patient. If the cryostat should come into contact with the patient, this would constitute a considerable risk for the patient.